1. Field of Invention
The present invention relates to an analog-to-digital converter (A/D converter). More particularly, the present invention relates to an automatic clamping A/D converter.
2. Description of Related Art
Presently, an analog-to-digital converter (A/D converter, “ADC”) is required in a digital circuit or a device for processing video signal, wherein a video signal is cross linked in AC to ADC, and then is sampled by the ADC to become a digital video signal. FIG. 1 illustrates a conventional video signal processing device 100. FIG. 2 illustrates the waveforms of the video signal and the clamping signal thereof. Referring to both FIG. 1 and FIG. 2, the video signal processing device 100 includes a capacitor 101, a switch 103, a digital-to-analog converter (D/A converter, “DAC”) 105 and an A/D converter (ADC) 107.
Wherein, one terminal of the capacitor 101 receives a video signal, while the other terminal thereof is coupled to the first terminal of the ADC 107 and the first terminal of the switch 103. The second terminal of the switch 103 receives a clamping signal CP, and the third terminal thereof receives a fixed voltage level Vfix. The DAC 105 receives an offset value Offset and outputs a DC offset level DL to the ADC 107. Finally, the ADC 107 outputs a digital signal ADC_out.
First, the capacitor 101 is used as a DC blocking capacitor to filter out the DC portion in the received video signal Video and to send the filtered video signal Video to the ADC 107. The switch 103 generates a clamping interval CPI while the clamping signal CP is logic 1, and the filtered video signal Video is pulled up to be above the fixed voltage level Vfix during the clamping interval CPI. Next, the DAC 105 receives an offset value Offset and outputs a DC offset level DL to the ADC 107 according to the offset value Offset. Finally, the ADC 107 converts the received video signal Video into a digital signal ADC_out according to the DC offset level DL output by the DAC 105.
While the video signal processing device 100 is processing the video signal Video, the digital signal ADC_out output by the video signal processing device 100 may not be identical to the offset value Offset, which will result in inaccuracy of the digital signal ADC_out output by the video signal processing device 100.
To solve the aforementioned problem, in the conventional technology, a microcontroller (MCU) is used to adjust the offset value Offset after analyzing the output digital signal ADC_out, and then to fix the offset value Offset when the offset value is optimized. Accordingly, the digital signal ADC_out output by the video signal processing device 100 is made identical to the offset value Offset, so that the digital signal ADC_out is more accurate.
However, misjudgment of the MCU may be caused because the MCU may not be able to detect the clamping interval CPI of the clamping signal CP, or other factors such as drift of the digital signal ADC_out caused by temperature increment of the video signal processing device 100, so that the digital signal ADC_out cannot be made identical to the offset value Offset; besides, adding an MCU will increase the manufacturing cost.